auto account activation, better startup experience, Julia v0.3.3

Account activation will not require an invite code any more. New account activation will however be subject to a configured rate limit. Beyond the set rate limit, new accounts will be activated offline and an e-mail intimation will be sent.

New session creation should be slightly faster now with:
- session creation initiated in authentication step, instead of waiting for a redirect from the browser
- loopback volumes are kept in a semi-prepared state by cleaning and pre-loading Julia packages at the time a disk is returned back to the pool.
- smoother loading animation (now using AJAX instead of refreshing the complete page)

Julia version is now 0.3.3.